This invention relates to microwave amplification, and particularly it relates to an apparatus and techniques for providing an amplifier capable of substantially continuous tuning between about 1 GHz and about 30 GHz with relatively flat gain and good impedance match. The invention is particularly useful in the realization of wideband microwave test equipment employing a signal source, such as a YIG-tuned oscillator.
Signal sources are presently available in microwave test equipment which cover frequency ranges of about 2 GHz to about 26.5 GHz in the form of sweep oscillators, signal generators and frequency synthesizers. Such signal sources are inherently very expensive because it has heretofore not been possible to design a single microwave microcircuit capable of generating signals over a broad frequency range with relatively flat gain and impedance. The normal approach for such instruments is to partition the frequency range over which operation is desired into relatively narrow-band segments based on the design frequency range of passive matching networks or active device gain regions and then to provide for each frequency range a linear amplifier and matching network specific to the segmented frequency range. The output of each device in a particular segmented frequency range is then typically routed through an electromechanical switch or a switching diode to a common output terminal.
In recent years, the availability of gallium arsenide metal semiconductor field-effect transistors (MESFETs) has made it possible to provide amplification of electrical signals in the microwave region up to about 40 GHz and beyond using only solid-state devices. Moreover, MESFETs can be operated as nonlinear devices producing harmonics if biased to operate in a nonlinear region. Dual-gate MESFETs in particular have been shown to exhibit pronounced nonlinearities which can be employed to advantage to provide high efficiency multiplication.
Unfortunately the input and output impedances of the GaAs MESFET have made it difficult to achieve single device wide bandwidth operation. Still further, harmonic operation of a device causes the generation of unwanted harmonics and other spurious signals which are not readily controlled in a wide bandwidth device, i.e., a device operative over more than about two octaves.